Functional Comfort

Suspension, a word built for automobile chassis and screaming motorcycles, matters more to a power wheelchair than one might think. It’s no accident that suspension systems are well debated in the motorcycle industry, after all. For these riders the difference between a “softail” and a “hardtail” is not only about comfort — it’s a defining feature. Are you part of the new generation, wanting a comfortable ride built for distance? Or are you old school, gravitating toward a rigid “hardtail” so that you can feel the road?

For power chair users, suspension systems can provide similar definition. The power chair might not be built for the highway, but its users never get off the bike. Turns out, the distance between “comfortable ride” and “clinically beneficial” is surprisingly short. The suspension system is pivotal to both. Mobility Management asked power chair suspension experts to tell us why.

Staying on Traction

Originally built as a bicycle with a motor, the motorcycle needed something its power-less ancestor did not have — a force to keep the tires in contact with the road. The power wheelchair — a chair with a motor — must have the same considerations. As such, the utilitarian purpose of suspension can be summed up as traction.

“In the absence of a suspension, the chair can ‘high-center.’ This is the condition where the drive wheel loses contact with the ground due to uneven surfaces. The suspension should allow the casters and drive wheel to move free enough for the drive wheel to remain in firm contact with the ground, ensuring mobility,” says Dan Critchfield, product manager for Sunrise Medical’s power line.

“The larger, wider and softer a tire is, the softer or ‘better’ the ride,” says Brad Peterson, VP of Professional Affairs and Clinical Education for Invacare Corp. “Power wheelchairs with pneumatic tires and casters provide a softer ride, but they require upkeep to maintain the optimal performance. And there are also concerns with durability and reliability. Furthermore, a larger front caster on rear-wheel-drive and mid-wheel-drive (chairs) will provide a better ride and more obstacle-climbing capability, but the trade-off is front-rigging positioning and caster interference.”

In fact, different suspension designs often require a trade-off, Peterson explains. But all suspension systems “work differently based upon drive-wheel position, geometry of the power base and the type of stability/performance that is desired,” he says.

Critchfield says, “all suspensions intend to add some type of cushion between what the terrain is providing and what the end user is actually experiencing.” For most, shock and vibration absorption comes from use of a swing-arm and spring design.

“The motors and casters simply pivot on the swing-arm and have some type of damping,” he explains. “This can be a coil spring or rubberized material like elastomer.”

Yet, “many bases mechanically connect front casters with drive wheels to provide downward force, traction and leverage for climbing. It is tough to balance stability and performance when using springs exclusively,” Peterson says. “A spring that is soft enough to ‘give’ and allow the chair to climb may be too soft to provide the needed stability, and springs can also be very sensitive to user weight and orientation.”

Peterson explains that traction doesn’t just come into play for larger obstacles.

“As manufacturers, we are often focused on how big the obstacles are (that) our systems can climb and safely descend. In talking with long-time end-users, their biggest frustrations are the smallest obstacles — cobblestone, pavers, brick walkways, the truncated domes you find at some crosswalks,” he says. “These types of surfaces deliver a constant, consistent vibration that is felt throughout the entire power base and elicits pain, fatigue and discomfort.”

In fact, Peterson says, many consumers are more concerned with negotiating a 1" threshold than a 3" obstacle.

“A good, balanced suspension will allow someone to navigate these smaller obstacles safely, smoothly, and with almost no jarring or movement of the seat. That is pretty important, powerful stuff, especially if the drive control is digital or not a standard joystick,” Peterson says.

On the flip side, some users will need more from the suspension system.

“That is why there are so many choices out there,” Peterson says. “It is so incredibly important for people to try the power wheelchair in their home environments before choosing. Everything handles great and smoothly on the perfectly smooth hallways in a rehab facility or clinic.”

As traction improves, the end user also experiences an additional benefit — independence.

Melissa Bourque, director of Market Development for Power Wheelchairs at Permobil, explains that traction prevents users from getting stuck, which is imperative for independence.

Jay Doherty, OTR, ATP/SMS, director of Clinical Education at Quantum Rehab, agrees. Power chairs enable the user to go outside, which puts them in the places their family and friends go, he says.

In addition, stability created through better traction helps users brake, accelerate and turn, Bourque explains. As a result, stability increases safety and confidence during driving.

The added functional bonus of the reduction of vibration is chair longevity.

“Suspension can also impact the durability of the wheelchair,” Doherty explains. “The less vibration and rattling of the wheelchair itself, the longer the equipment will hold up.”

So for power chairs, traction is essential for maintaining wheel-to-ground contact. However, traction also helps the user to improve driving capability, to remain as independent as possible and to own a product that lasts.

A Move Toward Functional Comfort

Technology has done much to solve the complications that come with “being confined to a wheelchair.” The power chair now functions as an additional appendage, helping people do more, see more and be more. But there’s still the issue that the user stays in the chair pretty much all day. If you’re sitting in one spot — especially if you are in motion — then comfort is, and should be, an issue.

Enter the suspension system.

When consumers talk about comfort, a lot of what they expect comes from the suspension itself.

“I live in Rhode Island,” Peterson says. “We are a tiny little state, and the running joke every spring is the preponderance of massive, automobile-eating potholes. From the interstate to the smallest side road, they are everywhere. Imagine your daily commute in your car without suspension. Now imagine spending all day driving in the same ‘suspension-less’ auto, 8 hours plus. How would you feel? Tired, sore, on edge, etc.?”

A suspension system can do much to eliminate this fatigue and pain for power chair users simply by factoring in user comfort and ride quality in the design process, Bourque says.

Reducing shock and vibration is key to increasing comfort.

Doherty explains that suspension systems “can reduce fatigue by impacting how much the individual feels bumps or vibration and can also assist in reducing tonal impact when the wheelchair is driving over objects or rough terrain.”

“In more technical terms, you could say that the suspension aims to isolate the seating structure from vibrations that are generated by the driving surface and preserve users’ energy to accomplish daily tasks,” Bourque adds.

“I know ‘comfort’ is a dirty word when people talk about function or clinical benefit. However, in my experience even the most clinically beneficial technology or power mobility system will not be of much use if it is not comfortable, as it probably will not be used,” Peterson says. “People that are not comfortable will seek comfort. That may compromise position or lead them to use older technology.”

A good suspension system is not all about comfort, however. It also provides several clinical benefits.

Circle back to vibration reduction. Vibrations have many clinical implications for power chair users.

“If the person operating the wheelchair is being jostled around while driving the wheelchair, we will typically see an impact on their tone,” Doherty says. “Often, this impact is increased tone because the movements and shifting of the wheelchair can initiate a tonal response from the individual.”

Peterson agrees. Extensor tone and spasticity will then lead to poor positioning and head control, which increases risk for pressure injuries and reduces access to drive controls, he says.

“Additionally, trunk control is compromised by consistent movement, rocking or larger ‘jolts’ that are unchecked by suspension,” Peterson says. “And these movements can also result in additional shear forces in vulnerable areas.”

In fact, “research has shown an increased risk of spinal damage associated with vibration forces transferred directly through the seating system to (the) user’s spine,” Bourque says. “Wheelchair users may also be more sensitive to vibration related to other factors, such as pain, increased tone and fatigue. Vibration forces can lead to a loss of position over the course of the day and contribute to shear forces along seated surfaces, which may increase risk for tissue breakdown.”

Doherty says that as the suspension system absorbs vibrations, it essentially stabilizes the seated position and increases the ability of the user to control the power chair.

This stability might feel a whole lot like comfort. After all, a user who doesn’t experience bumps can tolerate the seated position longer, Doherty says.

Cruising Along with Today’s Technology

Suspension has always been a part of power chair development, Doherty says. However, the way manufacturers look at suspension
systems has evolved over the years.

“For years suspension focused on climbing ability, but now with advancements in technology we can provide a suspension that climbs well and provides a smoother ride,” Doherty says. “This will allow us to design power wheelchairs with stability and increased comfort. Using a more dynamic suspension allows for the possibility of giving providers a choice of suspension based on weight or even suspension types.”

“In speaking with engineers here at Invacare, they all agree that consideration of suspension and ride quality for the user has made them think of better ways to manufacture chairs,” Peterson says. “A good suspension system makes a power wheelchair a more complex machine. Chairs need to have that great balance of performance and stability, be robust enough to endure a longer funding cycle, perform in a variety of environments on varied surfaces, be flexible enough to interface with different power positioning systems, and fit into more constrictive funding situations.

“Consumers are expecting more, and they should,” Peterson says.

Many manufacturers have focused on flexibility in design to provide the ultimate comfort for each client’s unique needs.

For example, Permobil has “increased focus on suspension performance,” Bourque says, which plays a significant factor in the design of the chassis.

“Critical parameters to be considered are geometry, structural rigidity and stiffness, reliability and durability, and robustness to promote a suspension system that works for all user weights, at all drive speeds and in a variety of seating positions,” Bourque says.

In addition, the suspension system is adjustable, which allows adaptation to the user’s weight and environment, Bourque says.

Quantum Rehab recently released a shock suspension system called SRS, which stands for Smooth Ride Suspension. The name itself illustrates a shift in focus for suspension. Doherty explains that the suspension system significantly reduces vibrations no matter what terrain or obstacle the user traverses.

“In addition, when the wheelchair drops down off a small curb, much of the impact is also absorbed by the suspension system,” he says.

For Sunrise, the newest suspension system in their line-up actually senses “when it’s appropriate to let the suspension move and when the suspension should not move,” Critchfield says. “These active suspensions provide additional stability to the end user when going down declines or off of a curb.”

Paying for the Upgrade

This might come as a bit of a shock. Suspension is so important that it is required on Group 3 and 4 power chairs, Critchfield says.

In fact, the classifications for power chairs and the reimbursable rate are very similar to motorcycles. A “hardtail” motorcycle does in fact imply a simpler design, no suspension and a reduced cost. Whereas, a “softail” provides comfort through better suspension, a longer ride and a higher price tag. In essence, a softail is a more complex machine, much like the Group 3 and 4 power chairs.

As such, this class of power chairs comes with certain design expectations that payors recognize.

“Every Group 3 power wheelchair must have some kind of vibration-dampening system in the wheelchair, as well as meet minimal climbing capabilities,” Doherty says. “Therefore, most manufacturers meet these requirements through use of a suspension system.”

Peterson says funding sources don’t directly pay for the suspension system per se.

“Simply stated, a Group 3 power wheelchair can climb a larger obstacle than a Group 1 or 2 can,” he says. “Suspension is a key component in determining how large an obstacle a chair can navigate.”

To get funding for a suspension system, the clinician and provider must justify the power base itself, Doherty says. Access to a power chair suspension system — no matter what type is chosen — ultimately improves the clinical condition of the user.

“Minimizing the exposure to vibrations is important to promoting a power wheelchair user’s overall health, enhancing functional endurance and quality of life,” Bourque says.

Fortunately, power chair suspension systems can define the end-user experience through a more comfortable ride, while enabling the user to get out on the highway and stay there longer. Functional comfort and adventure await!

This article originally appeared in the April 2018 issue of Mobility Management.